Non-invasive ventilation involves the delivery of fresh respiratory gases to a patient through a non-invasive means such as a mask, hood, or helmet rather than through an invasive means such as an endotracheal tube inserted via an oral, nasal, or tracheal opening in a patient. Continuous positive airway pressure (CPAP) ventilation and bi-level ventilation are two specific techniques of non-invasive ventilation. CPAP ventilation, as implied by the name, provides a continuous pressure of air during ventilation which maintains the airway in an open state and thus can fill the lungs with air thus requiring less work from respiratory muscles. CPAP is often used for patients with respiratory failure or near respiratory failure and for individuals with sleep apnea. Hi-level or variable level ventilation is often used for sleep apnea patients and for non-invasive ventilation for respiratory insufficiency or failure in institutional and home setting and is similar to CPAP, except that pressure is varied during inspiration and expiration. For example, pressure is lowered during expiration to ease exhalation. Compared with invasive ventilation, non-invasive ventilation can result in lower patient stress levels and lower trauma to patient airways. As such, non-invasive ventilation techniques offer more patient comfort than invasive ventilation techniques.
There are three major components involved in non-invasive ventilation: a ventilator which is an item of hardware which supplies fresh respiratory gas(es); a patient interface such as a mask; and a breathing circuit (i.e., tubes and connectors) that couple the ventilator with mask such that the fresh respiratory gases can be supplied to the patient for breathing. There are generally two techniques of non-invasive ventilation that are commonly in use: single limb, and dual limb.
Single limb breathing circuit applications involve blowing high flow levels of fresh respiratory gas into the patient interface, and relying on vent ports in the patient interface for allowing exhaled respiratory gases to exit the patient interface into the atmosphere. Vent ports or vents are designated leakage points that allow for a controlled leakage, or venting, of fresh respiratory gases in order to maintain a desired pressure of respiratory gases within a patient interface and to clear exhaled carbon dioxide. “Single limb” refers to the fact that a ventilation limb or limbs coupled with a patient interface only supply fresh respiratory gas and do not provide a return path for exhaled gases. As such, in some “single limb” applications a fresh respiratory gas supply tube may split into two or more tubes/limbs that allow fresh respiratory gas to enter a patient interface via more than one location. Because of the presence and reliance on vents, single limb non-invasive ventilation is also referred to as vented non-invasive ventilation.
Dual limb breathing circuit applications involve respiratory gases being blown into a patient interface via a first limb and exhaust gases being evacuated from the patient interface via a second, separate limb. Because of the second limb which is used for evacuation of exhaust gases, no vents are needed in the patient interface for venting exhaust gases into the atmosphere. Because no vents are required, dual limb non-invasive ventilation is also referred to as non-vented non-invasive ventilation.